a significant glance at his gun.

"Maybe it's some ob dem moon-men, what don't laik de idea ob us goin'
dere arter dere diamonds," volunteered the colored man.

"Perhaps," admitted Jack, with a smile. "But certainly some one has
been around here who had no business to be, and we must find out who it
was. Better take a look around, Wash."

"I'll help him," said Andy, and, with his rifle in readiness for any
intruders, the old hunter followed the colored man outside the big
shed.

Meanwhile Professor Roumann and Mr. Henderson were carefully examining
the exploded motor.

"I should have looked at the breech plug before turning on the power,"
said the German, "but I had no reason to suspect that anything was
wrong." He went on to explain that the explosion was something like
that which occurs when the breech-block of a big navy gun is not
properly in place. The force of the Cardite, instead of being directed
against the piston-heads of the motor, shot out backward, and almost
into the face of the professor, who was operating the machine.

"But what could be their object?" asked Mark. "Who would want to injure
us, or damage the projectile?"

"Some enemy, of course," declared Jack. "But who? The crazy machinist
is out of it, and as for that man who sent the note to you, he seemed
too big a coward to attempt anything like this."